The present invention relates generally to a pseudo stereophonic device for producing a pseudo stereophonic signal from a monophonic signal.
Examples of a pseudo stereophonic method for producing a pseudo stereophonic signal from a monophonic signal mainly include two methods; a comb filter system and a band division system.
(1) Comb Filter System
FIG. 5 illustrates the configuration of a pseudo stereophonic device employing the comb filter system.
The pseudo stereophonic device employing the comb filter system has the simplest configuration as a pseudo stereophonic device.
An input signal S is fed to a first adder 111 and a second adder 112, and is fed to a delay unit 101. A signal obtained by delaying the signal S in the delay unit 101 is fed to a multiplier 102, where the signal is multiplexed by a predetermined factor. An output of the multiplier 102 is fed to the first adder 111 and the second adder 112.
In the first adder 111, the output signal of the multiplier 102 is added to the input signal S, and the result of the addition is outputted as a pseudo left signal LOUT. In the second adder 112, the output signal of the multiplier 102 is subtracted from the input signal S, and the result of the subtraction is outputted as a pseudo right signal ROUT.
The longer a delay time allowed to the delay unit 101 is, the more a stereophonic feeling between the two output signals LOUT and ROUT is increased. However, the signal obtained by the delay is heard as an echo. Accordingly, a delay time of several microseconds is generally allowed to the delay unit 101.
If the delay time of the delay unit 101 is several microseconds, however, non-correlation between two channels is insufficient, so that the stereophonic feeling is insufficient. Particularly, the comb filter system is not suitable for two-channel reproduction processing of a multichannel signal using a sound image localization processing technique.
(2) Band Division System
FIG. 6 illustrates the configuration of a pseudo stereophonic device employing the band division system.
An input signal S is delayed by one sampling time period by each of a plurality of delay units D1 to Dm connected in series.
Pairs of multipliers ML1 and MR1 to MLm+1 and MRm+1 are respectively provided with respect to the input signal S and output signals of the delay units D1 to Dm. The input signal S and each of the output signals of the delay units D1 to Dm are inputted to the corresponding pair of multipliers, where they are multiplexed by a factor.
Output signals of the one multipliers ML1 to MLm+1 in the pairs of multipliers are added to each other by adders AL1 to ALm, and the result of the addition is outputted as a pseudo left signal LOUT. Output signals of the other multipliers MR1 to MRm+1 in the pairs of multipliers are added to each other by adders AR1 to ARm, and the result of the addition is outputted as a pseudo right signal ROUT.
The delay units D1 to Dm, the one multipliers ML1 to MLm+1 in the pairs of multipliers, and the adders AL1 to ALm constitute a first FIR (Finite Impulse Response) digital filter.
The delay units D1 to Dm, the other multipliers MR1 to MRm+1 in the pairs of multipliers, and the adders AR1 to ARm constitute a second FIR digital filter. The delay units D1 to Dm are shared between the first FIR digital filter and the second FIR digital filter.
The filter characteristics of the first FIR digital filter are shown in FIG. 7, and the filter characteristics of the second FIR digital filter are shown in FIG. 8. As can be seen from FIGS. 7 and 8, the filter characteristics of each of the FIR digital filters are such characteristics that a frequency band is divided into a plurality of pass and stop bands, and the pass bands and the stop bands alternately appear. The filter characteristics are such characteristics that the pass and stop bands in the first FIR digital filter and the pass and stop bands in the second FIR digital filter are opposite to each other such that the respective filter outputs LOUT and ROUT are not correlated with each other.
In the pseudo stereophonic device employing the band division system, if each of the pass and stop bands in each of the FIR digital filters is wide, the FIR digital filter may be only composed of hundreds of taps. However, sound is offset for each wide frequency band, so that an unnatural tone color is obtained. On the other hand, if each of the pass and stop bands in each of the FIR digital filters is narrowed, non-correlation is improved, so that a natural tone color is obtained. However, the FIR digital filter must be composed of not less than thousands of taps, so that a huge amount of processing is required.
As described above, in the pseudo stereophonic device employing the comb filter system, the processing is light, while sufficient non-correlation (stereophony) cannot be performed. In the pseudo stereophonic device employing the band division system, a huge amount of processing is required to perform sufficient non-correlation.
An object of the present invention is to provide a pseudo stereophonic device in which sufficient non-correlation can be performed, and a huge amount of processing is not required.
In a pseudo stereophonic device for producing a pseudo stereophonic signal from a monophonic signal, a first pseudo stereophonic device according to the present invention is characterized by comprising m delay units connected in series and gradually delaying an input signal S, m FIR digital filters for respectively subjecting output signals Sk (k=1, 2, . . . m) of the delay units to filter processing, and an operating circuit for executing, letting Yk (k=1, 2, . . . m) be outputs of the respective FIR digital filters, an operation expressed by the following equation (1), to produce pseudo stereophonic signals LOUT and ROUT:                                           L            OUT                    =                                    Y              1                        +                                          ∑                                  k                  =                  2                                m                            ⁢                              xe2x80x83                            ⁢                              Y                k                                                    ⁢                  
                ⁢                              R            OUT                    =                                    Y              1                        -                                          ∑                                  k                  =                  2                                m                            ⁢                              xe2x80x83                            ⁢                              Y                k                                                                        (        1        )            
The delay unit in the first row may be omitted, and the input signal S may be inputted to the FIR digital filter in the first row and the delay unit in the second row.
Letting nk be the number of taps composing the FIR digital filter in the k-th row, it is preferable that a filter factor of each of the FIR digital filters satisfies the condition expressed by the following equation (2):
Wk,i=Wmxe2x88x92k+2,nmxe2x88x92j+1xe2x80x83xe2x80x83(b).
A second pseudo stereophonic device according to the present invention is a pseudo stereophonic device equivalent to the first pseudo stereophonic device satisfying the foregoing equation (2), characterized in that one multiplier is shared between two multipliers, respectively having equal filter factors, in the different FIR digital filters.